Method and appabattts fob detekmining the contour of stjbtebbanean



March 19, 1929. B. M COLLUM Re. 17,242 IETHOD AND APPARATUS FORDETERMINING THE CONTOUR OF SUBTERRANEAN STRATA Original Eiled Aug. 14.1922 3 $heet$ $heet FIG. 2.

FIG. 1.,

FIG. 6.

INVENTOR. 4am

FIG. 4.

March 19, 1929. a. MQCOLLUM 7 Re. 17,242

IETHOD AND APPARATUS FQR DETERMINING THE CONTOUR OF SUBTERRANEANSTRA'I'A Original Filed Aug. 14. 1922 3 Sheets-Sheet 3 FIG. IO

muantoz Mamaa of their physical properties.

Reissued Mar. 19, 1929.

UNITED STATES PATENT OFFICE.

BURTON IcOOLLUM, OF WASHINGTON, DISTRICT 0] COLUMBIA, ABBIGNOB .lO KC-'COLL'UM GEOLOGICAL EXPLORATIONS, INC., A CORPORATION 01' DELAWARE.

METHOD AND APPARATUS FOR DETERMINING THE CONTOUR OF SUBTERRANEANBTRA'IA.

Original No. 1,672,495, dated June 5, 1928, Serial No. 581,886, filedAugult 14, 1922. Renewed March 5, 1928. Application for reissue fledJanuary 11, 1928. Serial '80. 881,744.

My invention relates to methods of determining the contour ofsubterranean strata or boundaries of geologic formations, and has amongits objects the study of the geological conditions at depths that cannotbe conveniently and economically reached by ordinary means. Inparticular, I have found that by the use of my invention it is possibleto determine the location of deposits of various ores, mineral oils, andother valuable materials. My invention depends on the well knownprinciple that if a sound wave be transmittted through the earth partialreflection of the wave takes place at t e boundary between any twomasses which differ in'respect to certain By properly utilizing thetransmitted and reflected waves I am able to determine accurately theloca-- tion. shape, and extent of such boundaries, which information isof great value for the purposes stated above. My invention is furtherdescribed in the following specification, reference being made to theaccompanying drawings.

Of the drawings:

Fig. 1 is a diagram showing the relation between the contour ofsubsurface strata and the occurrence of certain valuable mineraldeposits.

Fig. 2 shows the principle of methods that have heretofore beenunsuccessfully tried to accomplish the object here sought.

Figs. 3 and 4 are typical examples of records showing difiicultiesconfronting previous attempts to accomplish the results obtained by myinvention.

Fig. 5 shows in diagrammatic form a practical embodiment of myinvention.

Fig.6 shows a typical record obtainable through the use of my invention.

Fig. 7 shows in diagrammatic form the primiple of an acoustic shieldwhich I use to improve the character of the graphic records obtained inconnection with the application of my invention.

Fig. 8 shows a combination of sound receiving devices which I have foundparticularly valuable.

Fig. 9 shows an arrangement of portions of the apparatus for determiningthe velocity of sound in the earth.

Fig. 10 shows a preferred method of fixing the sound receiving device incontact with the earth.

Fig. 11 shows an improved form of a sound receivin device which isuseful in connection wit 1 my invention.

Figs. 12 and 13 show diagrammatic arrangements of microphonic deviceswhich I have found useful in connection with my invention.

For the sake of clearness and brevity my invention is described belowwith articular referenceto but one of its practica applications, namely,the location of deposits of mineral oil and natural gases. It willreadily be seen, however, that the method may be appliedto' determiningthe location of many other kinds of mineral deposits.

It is well known that in regions where deposits of oil or gas may beencountered the deposits are not distributed generally throughout thearea, but are highly localized in pools occupying a relatively smallportion of the total otential oil bearing area. The location of t iesepools is governed by a well known principle illustrated in Figs. 1. Inthis figure, (1) is the surface of the ground and (2) a densesubterranean stratum of irregular contour concave upward at (3) giving asynclinal fold, and convex upward at (4) giving an anticlinal fold. Itis well known that in a potential oil bearing region the oil and gasaccumulatelocally at (5) under the anticlinal fold (4), it being forcedupward into this osition by the heavier salt water stratum (5 beneathit. The problem of locating a pool of oil in a potential oil bearingregion is therefore, one of determining the location of these anticlinalfolds in the subterranean rocks. This latter, as stated above, is one ofthe objects of my invention.

Heretofore, numerous investigators have endeavored to determine thecontour of subterranean strata by the use of sound waves reflected fromthem, but up to the present time none of these methods has beensuccessful. Fig. 2 illustrates some of the fundamen-' tal diflicultiesthat have confronted all these previous attempts and prevented theirsuccessful application.

In their fundamental principles these methods have all comprised asource of sound (7 which has heretofore always been placed either on orbelow the snrface of the earth. Thetheory is that sound travels outradially in all directions and is in part reflected from the boundary 2,3, 4, 8, 10, and 12, the part.

of the wave incident at the point (8) being reflected to the point (9),that part incident at (10) being reflected to the point (11) and so on,the angle of reflection being equal to the angle of incidence. It isevident that if only this simple condition existed and if we couldclearly distinguish at any point of known position such as at (11) forexample,

between the direct transmitted wave (14) which goes either directly orthrough shallow subsurface strata to the point (11), and the wavereflected to the point (11) from the point (10)," we could by well knownmeans.

calculate the depth of the point (10) on the reflectin surface. Seriousdifficulties of a practice. nature prevent the realization of thissimple set of conditions. In the first place, the velocity of sound inthe rock layer (2) is practically always much greater than in thesurface strata. On this account when the slowly travelling sound wavereaches the nearest point, as at (3), of the reflecting rock layer, asound wave of relatively high velocity moves along the rock layer asshown by the arrows (15), and all the while a portion of the energyofthe wave is being diffracted upward into the overlying strata asindicated by the arrows (16), and this diffracted energy moves upwardand may reach the point (11) before the arrival of the true reflecteddiffracted disturbances will be detected at (11) which will completelyobscure the arrival of the true reflected wave.

Fig. 3 shows a typical record which reveals clearly the seriousness ofthis difficulty in practice. This is a record of disturbances receivedat the detector placed at a point cor responding to the point (11) dueto a single quick pulse of sound sent out from the source at the point(7). In consequence of the combined effect of the direct transmittedwaves, of which there are three distinct types, namely, a compressionwave, a transverse wave,

and a surface or Rayleigh wave, all of which travel at differentvelocities and therefore reach the detector at different times, andfurther, the innumerable diffracted waves due to the reaction on the twoformer by the subterranean reflecting surfaces as described above, therecord becomes so complex that the effect of the arrival of any purereflected wave is entirely obscured so that the recordis entirelyworthless for the purpose desired.

It. will be evident that this difficulty will be the greater the moreremote is the detector at the point (11) from the source (7), and

.that it can be diminished by placing the dctector close toth'e source.This isshown'by comparing Figs. 3 and 4. These two records are theresult of the same source of sound at- (7) but in Fig. 3 the receiverwas 150 feet from the source, while 1n Fig. 4 it was but 50 feet away.Interchangmg receivers give identical effects showing that thedifference in form is not due to the influence of the receivers. Thesensitivity of the recording instrument was, of course, adjusted to givesuitable sensitivit in the two cases.

Although tfiese diffraction effects may be thus diminished by bringingthe detector closer to the source, the disturbances produced by thedirect transmitted waves mentioned above become much more violent incomparison with the reflected waves so that if the distance is madeshort enough to substantially eliminate diffraction effects thetransmitted waves completely obscure the advent of any reflected waves.It is evident, therefore, that no location of the detector can be foundthat will permit it to distinguish definitely between the true reflectedwave and disturbances due to diffraction and direct transmission.Similar disturbances result in the case wave trains are used in lieu ofsingle pulses.

I have now invented a very simple expedient whereby .the foregoingtroubles can be entirely obviated. I accomplish thisend by placing thedetector or the source, preferably the latter, high up in the air and soarrange the two that the direction of the reflected waves reaching thedetector makes only a very small angle with the direction of thetransmitted waves. preferably not more than a few degrees. This angle ismade small, as in the arrangements hereinafter described, by causing thedistance, measured vertically between the shot or sound wave source andthe detector, great as compared with the horizontal distance between thedetector and the sound wave source or shot. By keeping this angle small,the diffraction disturbances are avoided and by placing the source at aconsiderable elevation above the surface of the earth the difficultiesdue to the direct transmitted wave are not only eliminated, but thiswave becomes very useful as will appear from the following detaileddcscription of the essential features of my in vention.

My invention will be clearly understood by reference to Fig. 5. Thesource of sound (17) is placedhigh up in the air. This source may be ofany suitable kind, but I prefer to use a short abrupt sound such as thatproduced by firing a charge of explosive llf) ' 1y simplifying therecord.

or by the sudden release of gas under pressure. Approximately below thesource (17 and either on.or slightl below the surface of the earth, Iplace a etector (18) which may be of any type, such use microphone,piezo-electric crystal, or electromagnetic detector. Wires extend fromthis detector to a recording device (19) of a type to record thedifl'erence in time between two or more events. The well knownoscillograph having constants adapting it to this particular work istypical of the recording devices which I have found suitable. It will beevident that'if a sudden sound be produced at the source (17) the wavewill travel downward and strike the surface of the earth (20) where aconsiderable part of the energy will be reflected and pass ofl' intospace. A part, however, will be transmitted to the earth and thisportion immediatelv produces an effect on the detector (18) w 1icl-1 isnear the surface and this eflect is recorded on the recorder (19). Thispoint on the record is then used as the zero of time to which subsequentrecorded events are referred. The wave then travels downward until itstrikes the first reflecting surface (21) where a part of its energy isreflected upward to the surface, where it again affects the detector,and the time elapsing between the arrival of the reflected wave. and

r the arrival of the transmitted wave will be determined. The velocityof sound in the overl ing stratum can be determined and the fipthofthe-surface from which reflection takes place can be readilycalculated from this velocity and the measured time interval between thearrival of. the direct transmitted wave and the reflected wave.' It willbe evident that if the depth of the reflecting surface be determined ata suflicient number of points thecontour of thissurface will be known.

It will be quite evident that with this arrangement of apparatus theeffects on the detector of both the Rayleigh wave and the transversewave in the earth will be eliminated, a'nd only those effects due to thecompression wave will be recorded in either the transmitted or reflectedwave. thereby great- Itwill also be very evident that all diflractioncfl'ccts, such as those described above. will not affect the detector.In consequence of this a very simple form of record, like that shown inFig. 6 is obtained where the ditl'crent events can be clearlydistinguished and the time intervals accurately measured.

A further consideration of very great practical importance has to dowith the relative intensity as shown by the record of the directwave,actuating the receiving device, and of the reflected wave coming backfrom the surface under investigation. It will be seen that the soundwave emanating from the source (17) travels out spherically in alldirections,

and the intensity of the wave at any point is goverened by the inversesquare law. Suppose, for example, that the height of the source 17 abovethe detector (18) is equal 1 to the depth of the reflecting surface(21). In that event when the sound wave reaches the detector (18) it hasa certain intensity. Suppose now that 100% ofthe energy of the wave isreflected from the surface (21). 'It will be evident that when thereflected wave front has travelled back again to the detector (18) thetotal distance which it will have traversed from the source (17) will bethree times as great as the distance traversed by the direct wave ingoing from the source (17) to the detector 18) The intensity of thereflected wave when it reaches the detector would therefore be onlyone-ninth of the in tensity of the direct wave. If, as is usually thecase in practice, only a fraction of the energy is reflected from thesurface (21) the intensity of the reflected wave becomes still furtherreduced. If new thesensitivity of the arrangement is made great enoughto give a sufliciently large effect due to the reflected wave, thedisturbances due to the direct wave will be so great that they mayinterfere seriously with the proper interpretation of the records. Itwill be evident, therefore, that in general will be necessary to takesteps to increase the amplitude of the reflected wave, relative to thatof the direct wave. I have devised several means of accomplishing thisresult, each and all of which comprise a part of my invention.

One of the means whereby I increase the intensity of the reflected waverelative to that of the direct wave, is by putting a source of soundvery high up in the air as compared to the depth of the stratum underinvestigation. As seen from the example given above, if the depth of thestratum is substantially equal to the height of the source, thenassuming 100% reflection the intensity of the reflected wave at thereceiver will be only one-ninth of the intensity of the direct wave.Suppose, however, that the source be put to a height above the detectorof say five times the de th of the reflecting stratum under study. Inthat case the reflected wave travellltl ling back to the detector willhave travelled about 40% farther from the source than the direct wave.when the two pass the detector. Applying the inverse square law it willbe seen that in this case, assuming reflection, as before. the intensityof the reflected wave at the detector will be 1/1.96. or approximatelyone-half of that of the direct wave, as compared with the ratioone-ninth, when the source is placed at the lesser elevation. It willtherefore be seen that by putting the source very high in the air incomparison with the depth of the stratum under investigation, it ispossible, because of the inverse square law of propagation of soundcritical height of the source 17 of the sound energy utilized whichunder all circumstances is to be exceeded, nor is it necessar to knoweither the height of the source 1 nor the depth of the reflectingstratum. In actual earth all around it.

practice the procedure follows:

- A sound'wave is produced at any convenient height, as by a chargeexploded, say, 1,000 or 2,000 feet above the earths surface, and asuitable record, as photographic, 'is taken of the waves actuatingorinfluencing the detector. If upon examination of the record so takenthere is revealed a reflected event clearly distinguishable from theafter effects of the direct wave, it shows that the explosion occurredat a sufficient height. The significant fact is the time intervalbetween is substantially as the arrival at the detector of the directand reflected waves, and it is only necessary to know this timeinterval, which, when multiplied by the velocity of sound in theoverlying medium, gives a distance which is twice the depth of thereflecting stratum. If, on the other hand, the record shows no reflectedevent clearly distinguishable from the after effects of the direct wave,it is proof that the source of the sound energy was not sufficientlyhigh above. the detector, and in such case it is only necessary to takeanother record with the source of sound at a greater elevation.

A second means whereby I secure an increased ratio of the intensity ofthe reflected and direct waves,is by the use of 'an acoustic shieldinterposed between a source and the detector. One form of this is shownin Fig. 7. The acoustic shield (22) which can be made up in any form tobe substantially sound proof, is placed between the source (17) and thedetector (18) and preferably close to the latter. In practice I preferto put the shield (22) near or on the surface of the earth. as

shown in Fig. 7. It will now be seen that the sound energy travellingdownward from the source (17) strikes the shield and the The shield (22)may be designed either to reflect or absorb the energy striking it, inwhich event it .will be seen that no sound energy travels directly intothe earth at the detector (18). region all around the shield, the energypasses downward into the earth as will readily be seen, and is graduallydiffracted inward underneath the shield into the region (23).

'By the time the reflected wave from the surface (21) reachesthe'detector (18) the diffraction will have been sufficient to giyenearly a uniform dlstribution of energy 1n However, in the the reflectedwave, and the detector will therefore be actuated by the reflected wavewith nearly as much intensity as if the acoustic shield (22) did notexist. At the same time there will be very little effect due to thedirect wave, since only a very small amount of the energy of the directwave will be diffracted directly from the edges of the acoustic shieldto the source (18) I have found that in this way I can reduce theintensity of the direct wave at the receiver to a small fraction of whatit would be without the shield, and at the same'time secure nearly asmuch effect on the detector from the reflected wave as if the shield didnot exist.

A third method which I have devised for reducing the amplitude of thedirect wave in comparison with that of the reflected wave is shown inFig. 8. .It is Well known that because of the very great difference inthe acoustic properties-of the earth and air, a

Jenergy reflected at the surface of the earth bacli again intov the airand off into the atmosphere. Similarly, that part of the. energy whichgoes into the earth and is reflected back toward the surface from therefleeting surface (21), will on arrival at the surface be againreflected downward, only a small fraction of its energy returning againto the air. By taking. advantage ofthis principle I am able to reducethe intensity of the effect of the direct wave on the detector to anydesired degree without materially reducing the intensity of thereflected disturbance which it is desired to record. This isaccon'lplished by the use of two receiving devices as shown in Fig. 8.Here one receiving device (18) is placed in the earth as previouslydescribed, in which case it is actuated only by that part of the soundenergy passing into the earth. The second receiving device (24) isplaced to be responsive to the direct air wave to a much greater degreethan to the reflected ground wave, and very close to the detector (18).In order to make clear the method offnnctioning of this arrangement, letus assume that the sensitivity of the detector (24) bears to thesensitivity of the detector (18) the same numerical ratio as the soundenergy transmitted to the earth bears to the total sound energy incidenton the surface of the earth from the source (17). In that case it isobvious that the total effect produced on the detector (24) will be justequal to the total effect produced on the detector (18), dueto thedirect wave coming from the source (17). Consider now what happens whenthe reflected wave arrives again at the surface after having beenreflected from the subsurface (21) This wave travelling in the earthgive full effect on the detector (18) embedded in the earth, but onreaching the surface nearly all of its energy is again turned back in adownward direction, only a small fraction of it being transmitted to theair where it can affect the detector (24) It will be seen, therefore,that the eflect of the reflected wave will be enormously greater on thedetector (18) than it is on the detector (24) whereas the effect of thedirect wave on the two detectors will be substantially equal. If now thetwo detectors (l8) and (24) are coupled together in such manner thatthey tend to neutralize each other as regards their effect on therecording device, then the direct wave will produce no effect on therecords provided the two detectors are adjusted to give equal andopposite impulses, Whereas the reflected wave will be recorded throughthe dc tector (18) at almost its full value. In practice I prefer not tocompletely eliminate the direct wave on the record so that I do notadjust the detectors (18) and (24) so that they exactly neutralize eachother. I prefer to adjust them so that the resultant efi'ect of the two,due to the direct wave, is only a small fraction of the effect producedon either in-.

strument alone, as this gives an indication on the record showing thetime of arrival of the direct Wave, which is useful as a basis ofreference for the time scale. It will be seen, therefore, that by properadjustment of the relative sensitivity of the two detectors in Fig. 8,the relative intensity as shown on the record of the direct andreflected waves can be controlled to any desired extent. In practice anyone of the above described means for controlling the relative intensityof the effects of the direct and reflected waves may be used, or any twoor all of them may be used in combination if desired.

In order to measure the velocity of sound in the stratum between thesurface of the earth and the reflecting surface under investigation Iplace two receiving devices in the earth as shown in Fig. 9. one (18) ata suitable distance below the surface, and the second (25) a knowndistance below it, substantially in line with the direction ofpropagation of the sound wave. The difference in time of arrival of athe sound wave at the two receivers is measured by means of a recorderfrom which,

and the known distance between the receivers, the velocity is readilyobtainable. In some cases where there is reason to believe that thevelocity of sound in the overlying stratum may vary with depth, severalindicating devices may be placed at various depths in orderthat the lawof varlation of velocity with depth may be determined.

I have found that in order to secure a good sensitivity in theindicating devices and also in order to eliminate spurious disturbancesdue to vibrations of receiving devices themselves, it is desirable tohave the microphones very firmly fixed in contact with the earth.Thiscan be done by making a hole, placing the mlcrophone in 1t filledeither with earth of other suitable binding material and thoroughlytampin the filling material in place around and a ove the detector. Thisprocedure, however, is diflicult and time consumlng and renders verydiflicult the recovery of the indicating device, especially when buriedto a considerable depth, after the records have been taken.

I have devised a very simple and convenient means of firmly attachingthereceiving device to the earth which eliminates these troubles. Thisis shown in Fig. 10, where the recelvlng device is mounted inside of arigid case (26) which may be of metal or other suitable material. In thebase of this case is firmly attached a large screw (27), suitable forscrewing into the earth. To place a receiving device in position I firstbore a small hole, large enough to accommodate the recelver andextending to the desired depth, after which the receiver is placed downin the hole with the screw downward and by means of a suitable longhandled wrench the receiver case is turned so as to drive the screwfirmly into the earth. After the records have been taken the receivercan readily be unscrewed from its position and brought to the surface.As stated above, any one of the usual types of receiving devices may beused. I have found, however, that instead of using a single receivingelement it is often desirable to use a considerable number of suchelements grouped in a single unit in order to increase the sensitivityand reliabilit of the receiving apparatus. This is particu arly true incase carbon microphones are used as receiving devices. Thesemicrophones, when used singly exhibit certain inherent instabilitiesfrequently called frying, which gives rise to more or less erraticpulsations of current flowing in the microphone, which in turn producesdisturbances on the record, especially where a very sensitive recorderis used. This trouble is especially serious if one attempts to use avery large current in the microphone in order to increase thesensitivity. This difficulty can be greatly minimized by using a largenumber of microphone elements connectedin parallel, but such a simplearrangement cannot be used in practice. It is well known that in orderto use a microphone successfully and secure good sensitivity indetecting disturbances of relatively low frequencies, it is necessary touse it in conjunction with a mutual inductance having an iron core, andfurther, that the current flowing through the primary of this mutualinductance, which of course is the current flowing through themicrophone, must be kept small enough so as not to produce saturatlon inthe iron core. This fact placesa limit on the number of microphones thatcan be used in parallel on a single mutual induct-ance, and with theusual forms of inductance practically nothing is gained by the use ofmore than one or two microphones in this way. I have, however, devisedan arrangement whereby the ordinary forms of iron core mutual inductancemay be used effectively with a large number of microphone elements inproper combination. i

The essential elements are shown in Fig. 11. Inside the receiver case(26) is mounted a rigid plate (28), preferably tilted at an angle withrespect tothe axis of the case (26). I prefer to make this angle between30 and 60, but larger or smaller angles may be used if desired. Aterminal of each of the microphone elements (29) and (30) is generallyattached to the plate (28), and interposed between this plate and theother terminal of each microphone is placed a cushion of suitablefabric, such as cloth or other material, to serve as a damping agent topre-' vent vibrations in the microphone when it is actuated. Any desirednumber of such pairs of microphone elements may be mounted inside thecase (26). The receiver case is fixed to the ground with itsaxis in thedirection of the earth displacement which it is sought to record, inthis case being vertical. It will be evident that when the earthvibrates due to the passage of a sound wave or pulse, the receiver caseis moved up and down with the earth while the heavy case of themicrophone elements (29) and (30) tend to stand practically stationary.In consequence of this, it will be seen that the pressure on themicrophone elements (29) and (30) will vary as the wave passes, thuscausing vibrations in their resistance. It will be noted that when thepressure on the microphone element (29) is increased due to thedownwardmovement of the case (26), the microphone (30) will decrease so that thepulsations of resistance on the two microphone elements will beopposite. In order to make the elfects of the two groups cumulative onthe recording instrument, either of two arrangements may be used, one ofwhich is shown in Fig. 12. Here all of the microphone elements (29),(29) etc., which are similarly mounted with respect to the plate (28),are placed in one arm of a Wheatstone bridge while all those (30), (30),etc., which are so mounted as to give resistance variations opposite tothe ones in group (29) (29) etc., are placed in the adjacent arm of thebridge. It will be obvious that as the resistance of one group increasesand that of the other decreases, the two effects are cumulative indisturbing the balance of the bridge, and therefore in effecting theindications of the oscillograph or other instrument (19') coupled acrossthe diagonal of the bridge. The mutual inductance (32) may or may not beused, as deprimary coils (34) and (35) without'danger of saturating themagnetic circuit. When the current in one circuit lncreases While thatin the other decreases, the effects are cumulative in causing changes inthe ma netiza- 111011 of the iron core (33), and hence 1n actuating theoscillograph (19), which is connected to the single secondary coil (36).'As here shown, the microphone elements (29), (29), etc., are grouped inseries. It will be evident that parallel or series multiple grouping maybe used with equal effect, provided the number of turns in the primarycoils (34) and (35) of the mutual inductance are made to correspond tothe number of microphone elements in series.

A careful consideration of the foregoing discussion reveals that one ofthe fundamental features of my invention comprises the placing ofasource of sound and a receiver in such relation toeach other and to thereflecting surface, the depth or contour of which is to be studied, thatthe angle between the direct transmitted and the reflected wavesaffecting the receiver is small, whereby the disturbance due to thesurface waves, transverse waves, and the innumerable diffraction effectsabove discussed, are made to disappear. This might, of course, be doneby placing both source and receiver in the earth,

provided one is placed at a considerable depth, in order to have thereceiver remote from the source. It is, however, very difficult,expensive, and time consuming to place the instruments at a great enoughdepth to be effective. Furthermore, experience has shown that if thesource be placed in the earth the available sources of a quick, sharppulse, such as the firing of a charge of explosive, produce a violentdisruptive effect in the earth immediately surrounding it, which in turntends to change the character of the disturbance from a quick, simplepulse to a complex and greatly prolonged disturbance, thus defeating theobject of the arrangement. I'have found, however, that if the source ofsound be placed high up in the air, preferably high enough so that thewave front striking the earth will be practically a plane wave, thisdifficulty will be entirely avoided. If the wavefront striking the earthbe nearly plane, the subsequent diminution of intensity with distance,both before and after reflection, will be relatively slight so that theratio if the source be placed high up in the air, the intensity of theshock imparted to the earth at any point may be very slight, and nowheresufficient to cause permanent deformation of the medium, and still givea reflected wave of ample intensity for detection. On the other hand, ifthe source be placed on the surface or imbedded withinthe earth, theintensity of the shock at points very close to the source must be verygreat in order that the reflected wave may be of sufiicient intensity,and in practice it is found that permanent deformation of the earth veryclose to the source always occurs, thus giving rise to the increasedcomplexity and prolongation of the wave above described. It willtherefore be apparent that the placing of the source up in the air at aconsiderable distance from I the earth, as hereinabove described, is offundamental importance in eliminating certain of the practicaldifficulties that have heretofore been encountered in attempting toexplore subterranean strata through the medium of sound waves. Anysuitable means may be used for placing the source at a proper elevation.Where circumstances are such that a height of not more than about 100feet is sufiicient, a light telescoping pole or tower can be usedsuccessfully. As a rule, however, I have found that it is desirable toplace the source at a considerably greater elevation, and when this isdesired some other means can be conveniently used for putting the sourceup in the air. Any oneof a number of devices may be used if desired,such as a captive balloon, a kite, an airplane, or recourse may be hadto projecting a charge of explosive into the air, the same being firedby a time fuse in accordance With principles well known to militaryballistics.

For the sake of brevity in the appended claims, the term aperiodic asapplied to the sound produced by the source includes an abrupt soundwave or a sound wave 1mpulse or rapidly decadent sound waves, producedby a shot, explosion or equivalent means herein described, asdistinguished from sustained, continuous or undamped sound waves.

I claim:

1. The method of determining the contour of a subterranean stratum whichconsists of sending out a sound wave from a source of sound, causing thesaid sound wave to be transmitted through the earth to the saidsubterranean stratum and reflected therefrom, measuring the timeinterval elapsing between the passage of the said sound wave over aknown pointat a distance from said source and the passage of thereflected wave over the same point, measuring the velocity of sound inthe medium between the said known point and the said subterraneanstratum and calculating the distance between the said known point andthe said subterranean'stratum from the said time interval and the saidvelocity, the said source and the said known point being so placed withrespect to the said subterranean stratum that the path traversed by thedirect wave is substantially identical with the path traversed by thereflected wave.

2. The method of locating a subterranean stratum, which comprisesproducing an aperiodic sound wave, causing said wave to be transmittedthrough the earth to the subterranean stratum and to be reflectedtherefrom, measuring the time interval elapsing between the passage ofsaid wave past a known point and the passage of the reflected wave pastthe same point, determining the velocity of sound in the medium betweensaid known point and said stratum, and determining the distance betweensaid known point and said stratum from said time interval and saidvelocity, the place of production of said sound wave and said knownpoint being so positioned with respect to said stratum that the pathstraversed by the direct and reflected waves are substantially identical.

3. In the art of exploring subterranean regions, the method whichcomprises producing sound at a distance above the surface of the earth,and detecting at a point whose distance horizontally from the source ofsaid sound is small compared with its distance vertically therefrom, thesound reflected from a subterranean formation.

4. In the art of exploring subterranean regions, the method whichcomprises producing sound at a substantial distance above the surface ofthe earth, and detecting the sound reflected from a subtcrrrmcan fiirma;

tion at a point through which both the direct and reflected waves pass.

5. In the art of exploring subterranean regions, the method whichcomprises producing sound at a distance above the surface of the earth,and detecting, at a point adjacent the earths surface and whose distancehorizontally from the source of said sound is small compared with itsdistance vertically therefrom the sound reflected from a subterraneanformation.

6. In the art of exploring subterranean regions, the method Wl11Cl1comprises prod ucing sound at a distance above the surface of the earth,and detecting at a point adjacent the earths surface the sound reflectedfrom a subterranean formation, said point being located adjacentsubstantially identical paths in which the direct and reflected wavesare transmitted.

7. In the artof exploring subterranean ing an aperiodic sound wave at aista'nce above the surface of the earth, and detecting, at a point whosedistance horizontally from the source of said sound is small comparedwith its distance vertically therefrom the sound reflected from asubterranean formation.

8. In the art of exploring subterranean regions, the method whichcomprises producing an aperiodic sound wave at a distance above thesurface of the earth, and detecting, at a point adjacent the earthssurface and whose distance horizontally from the source of said sound issmall compared with its distance vertically therefrom the soundreflected from a subterranean formation.

9. In the art of exploring subterranean regions, the method whichcomprises producing sound at a distance above the surface of the .earth,and detecting, at a point whose distance horizontally from the source ofsaid sound is small compared with its distance vertically therefrom thesound transmitted to and reflected from a subterranean formation.

10. In the art of exploring subterranean regions, the method whichcomprises producing sound at a distance above the surface of the earth,and detecting the sound transmitted to and reflected from a subterraneanformation at a point adjacent substantially identical paths over which.the direct and reflected waves are transmitted.

11. In the art of exploring subterranean regions, the method whichcomprises producing an aperiodic sound wave at a dlstance above thesurface of the earth, and detecting, at a point whose distancehorizontally from the source of said sound is small compared with itsdistance vertically therefrom the sound transmitted to and reflectedfrom a subterranean formation.

12. In the art of exploring subterranean regions, the method whichcomprises producing an aperiodic sound wave at a distance above thesurface of the earth, and detecting the sound transmitted to andreflected from a subterranean formation at a point adjacentsubstantially identical paths over which the direct and reflected wavesare transmitted.

13. In the art of exploring subterranean regions, the method whichcomprises transmitting sound from a source to a subterranean formationand reflecting it therefrom, producing an effect by the direct soundwave, producing a'second effect by the reflected sound wave at a pointWhose distance horizontally from said source is small compared with itsdistance vertically therefrom, and

producing a composite indication by said effects.

14. In the art of exploring subterranean regions, the method whichcomprises transmitting sound originating at a dlstance above the earthto a subterranean formatlon to be earth to a subterranean formation toreflected therefrom, producing a pl alit f v effects by the direct andreflected sounds, and producin a composite indication by said efects,sai effects being roduced at pointsv direct and reflected sound,Indicating means,

and means for causing said detectors to affect sald indicating means inopposite senses.

,16. In a system of the character described,

means for transmitting sound through the e reflected therefrom, aplurality of detectors affected to greater extents respectively, bydirect and reflected sound, indicating means, and means for causing saiddetectors to affect said indicating means in opposite senses, saiddetectors disposed adjacent substantially identical paths over which thedirect and reflected sound is transmitted.

17 In a system of the character described, means for transmitting soundthrough the earthto a subterranean fornriiti on to bereflectedtherefrom, a plurality of detectors affected to greater extents,respectively, by the direct and reflected sound, and indicating meanscontrolled by said detectors, the source of said sound disposed at adistance above the surface of the earth.

18. In a system of the character described, means for transmitting soundthrough the earth to a subterranean formation to be reflected therefrom,a plurality of detectors affected to greater extents, respectively, bydirect and reflected sound, indicating means, and means for causing saiddetectors to affoot said indicating means in opposite senses, the sourceof said sound disposed at a distance above the surface of the earth.

19. In a system of the character described, means for transmitting soundthrough. the earth to a subterranean formation to be reflectedtherefrom, aplurality of detectors affected to greater extents,respectively, by the direct and reflected sound, and indicating meanscontrolled by said. detectors, the sound produced by said meansconsisting, of an aperiodic sound wave.

20. In a system of the character described, means for transmitting soundthrough the earth to a subterranean formation to be re flectedtherefrom, a plurality of detectors affected to greater extents,respectively, by the direct and reflected 'sound, indicating means, andmeans for causing said detectors to affect said indicating means inopposite senses, the sound produced by said means consisting of anaperiodic sound wave.

21. In a system of the character described,

I means for transmittin sound through the 22. In a system comprising adevice for determining the contour of a subterranean stratum andcomprising a source of sound, sound detectors, and a sound measurmgdevice, the method which comprises placing said source in the air abovethe surface of the "earth, disposing certain of the sound de- 7 tectorsin contact with the earth at points intermediate said sound source andthe subterranean stratum, and substantially in line with the normalextending from the subterranean stratum through said source, and placingother of the sound detectors so as to be actuated substantially only byan air wave, and so associated that its effect is opposed to that ofsound detectors in contact with the earth, and coupling the timemeasuring device to said detectors in such manner as to measure the timeinterval elapsing between the arrival ofa direct sound wave at thedetectors in earth and air, and of one or more reflected sound waves atthe detectors in earth.

23. In a system for determining the contour of a subterranean stratumand compris ing a source of sound, sound detectors, and a time measuringdevice, the method which comprises placing the source of sound in theair above the surface of the earth ata dis tance greater than the depthof the subterranean stratum, placing the detectors in contact with theearth substantially in line with the normal extending from thesubterranean stratum through said source, and coupling the timemeasuring device to said detectors to measure the time interval elapsingbetween the arrival of successive sound waves at said detectors. o

24. A system for determining the contour of a subterranean stratumcomprising a source of sound, a time recording device, microphonlcdevices, and means for mounting said m1crophon1c devices to efi'ectopposite phase relation-of pulsating change of their resistances intheir efi'ect upon said recording device.

25. A system for determining the contour of a subterranean stratumcomprising a source of sound, a time recording device, microphonicdevices electrically connected in parallel, a differentially woundtransformer having primary coils connected respectively in series withsaid microphonic devices and a secondary connected to the time recordingdevice, and means for mounting said microphonic devices to effectopposite phase relation of the pulsations of their resistances in theireffect upon said recording device.

26. In the art of exploring subterranean regions, the method whichcomprises transmitting sound to a subterranean formation to efiectreflection therefrom, producing a plurality of eflfects at pointsadjacent sub stantially identical paths over which the direct andreflected sounds are transmitted, and producing a composite indicationby said efiects.

BURTON MoCOLLUM.

